The Multiplayer Plugin together with the Signaling server by Scirra is a Peer to Peer connection, where the Signaling server only establish the connection between peers, so realtime Multiplayer. PHP would make a asynchronous Multiplayer, so you have to use AJAX. The third would be a server with Websocket, that would be realtime or asynchronous Multiplayer, depending on you server code. I made a dedicated server with the Multiplayer Plugin, but didn't post a tutorial yet. Or you use a thirdparty plugin like "Photon".

Witch one works best for your game have you to decide. There are Tutorials for all these variants, but Websocket is a bit more complicated.